Abstract
Abstract. Black carbon (BC) is known to have major impacts on both climate and human health and is therefore of global importance, particularly in regions close to large populations that have strong sources. The size-resolved mixing state of BC-containing particles was characterised using a single-particle soot photometer (SP2). The study focusses on the Indo-Gangetic Plain (IGP) during the pre-monsoon and monsoon seasons. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights during the pre-monsoon (11 and 12 June) and monsoon (30 June to 11 July) seasons of 2016. Over the IGP, BC mass concentrations were greater (1.95 µg m−3) compared to north-west India (1.50 µg m−3) and north-east India (0.70 µg m−3) during the pre-monsoon season. Across northern India, two distinct BC modes were recorded; a mode of small BC particles (core diameter <0.16 µm and coating thickness <50 nm) and a mode of moderately coated BC (core diameter <0.22 µm and coating thickness of 50–200 nm). The IGP and north-east India locations exhibited moderately coated black carbon particles with enhanced coating thicknesses, core sizes, mass absorption cross sections, and scattering enhancement values compared to much lower values present in the north-west. The coating thickness and mass absorption cross section increased with altitude (13 %) compared to those in the boundary layer. As the monsoon arrived across the region, mass concentration of BC decreased over the central IGP and north-east locations (38 % and 28 % respectively), whereas for the north-west location BC properties remained relatively consistent. Post-monsoon onset, the coating thickness, core size, mass absorption cross section, and scattering enhancement values were all greatest over the central IGP much like the pre-monsoon season but were considerably reduced over both north-east and north-west India. Increases in mass absorption cross section through the atmospheric column were still present during the monsoon for the north-west and central IGP locations, but less so over the north-east due to lack of long-range transport aerosol aloft. Across the Indo-Gangetic Plain and north-east India during the pre-monsoon and monsoon seasons, solid-fuel (wood burning) emissions form the greatest proportion of BC with moderately coated particles. However, as the monsoon develops in the north-east there was a switch to small uncoated BC particles indicative of traffic emissions, but the solid-fuel emissions remained in the IGP into the monsoon. For both seasons in the north-west, traffic emissions form the greatest proportion of BC particles. Our findings will prove important for greater understanding of the BC physical and optical properties, with important consequences for the atmospheric radiative forcing of BC-containing particles. The findings will also help constrain the regional aerosol models for a variety of applications such as space-based remote sensing, chemistry transport modelling, air quality, and BC source and emission inventories.
Highlights
Increased anthropogenic emissions into the atmosphere, especially over south Asia, have led to severe air quality issues (Schnell et al, 2018)
The analysis presented below first outlines the Black carbon (BC) characteristics within the boundary layer from the straight-level run (SLR) measurements during the pre-monsoon-to-monsoon transition period across northern India, followed by the vertical profile analysis (Sect. 3.1)
Our observations show that the BC mass concentrations are less than half over NE India than those are in the central Indo-Gangetic Plain (IGP) during the pre-monsoon season (0.70 and 1.95 μg m−3 respectively), a spatial pattern consistent with Thamban et al (2017)
Summary
Increased anthropogenic emissions into the atmosphere, especially over south Asia, have led to severe air quality issues (Schnell et al, 2018). Strong light absorption from BC coupled with its longer atmospheric lifetime makes BC an important aerosol constituent affecting regional climate (Bansal et al, 2018). Regional BC emission inventories have considerable variability owing to the emission factors, unreliability of fuel consumption estimates, and general lack of detailed in situ measurements (Bond et al, 2013; Rana et al, 2019). The BC mixing properties are of importance for further study and clarification across India, especially northern India where BC mass concentrations are known to be significant (Bond et al, 2013; Brooks et al, 2019)
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